Though an indexable rotary cutting tool having cemented carbide inserts detachably attached to a tool body can cut a work with high efficiency, the life of the insert decreases by the premature wearing of rake faces and flanks of cutting edges and the chipping and breakage of cutting edges due to a cutting load. Particularly in the cutting of difficult-to-cut materials such as stainless steel, heat-resistant alloys such as Ni-based alloys (for example, Inconel®), Ti alloys, etc., inserts suffer extreme decrease in life. To increase the life of inserts, various improvements of inserts have been proposed.
JP 2003-260607 A proposes an insert having sufficient cutting edge strength with reduced cutting resistance, which comprises as shown in FIG. 25, nose portions 112 at corners of an insert body having a substantially parallelogramatic plate shape, and long-side cutting edges 115 and short-side cutting edges 116 formed along ridgelines of a pair of adjacent flanks 113, 114 on both sides of each nose portion 112 and an upper surface, each long-side cutting edge 115 being inclined toward a bottom surface 117 as separating from the nose portion 112, and having a gently projecting circular cutting edge 118 and a straight cutting edge 119 in this order from the nose portion 112, and the projecting circular cutting edge 118 having an upper flank 121 and a lower flank 120. Because the lower flank 120 has larger clearance angle than the upper flank 121, a sufficient clearance angle can be secured without decreasing the strength of the projecting circular cutting edge 118 to which a large cutting load is applied. However, because the entire long-side cutting edge 115 has a cutting edge comprising the circular cutting edge 118 and the straight cutting edge 119, sufficient strength cannot be secured particularly in the straight cutting edge 119. In addition, finishing the projecting circular cutting edge 118 with high precision needs a high machining cost.
JP 2008-213078 A proposes, as shown in FIG. 26, an insert comprising an insert body 201 having a substantially parallelogramatic plate shape having rake faces 202 and flanks 204 on upper and side surfaces, gently outward curved main cutting edges 207 each extending between the rake face 202 and the flank 204 from a corner cutting edge 206 to a corner portion 209 having an obtuse angle, a fixing hole 205 penetrating from the upper surface to the lower surface for receiving a cramping screw, and auxiliary cutting edges 208; each flank 204 having a twisted surface portion 204A, a concaved portion 204B and a flat surface portion 204C in this order from the main cutting edge 207; and the concaved portion 204B having a radius of curvature larger in a center portion than in the corner to increase the strength of the cutting edge 207. In this insert, the main cutting edge 207 has a gradually increasing clearance angle by the twisted surface portion 204A, as going from the corner cutting edge 206 to the obtuse corner portion 209. However, such main cutting edge 207 does not have sufficient strength, because it does not have a sufficient clearance angle near the corner cutting edge 206 to which the largest cutting load is applied. In addition, finishing the outward curved main cutting edge 207 with high precision needs a high machining cost.
JP 2006-75913 A proposes, as shown in FIG. 27, an insert comprising main cutting edges 309 and auxiliary cutting edges 310 each connected to one end of each corner cutting edge 307 of a substantially equilateral-triangular, plate-shaped insert body 301; each main cutting edge 309 having a rake face 306 in an upper surface 302; each auxiliary cutting edge 310 having a flank 304 on a side surface; each main cutting edge 309 being constituted by first and second straight cutting edge portions 309A and 309B; and each auxiliary cutting edge 310 being constituted by a third straight cutting edge portion, whereby pluralities of straight cutting edge portions obtained by the main cutting edges 309 crossing with obtuse angles are in a bent-line shape. With such a shape, the entire length of cutting edges can be made shorter, suppressing the decrease and variation of cutting resistance, and dividing chips at intersections of straight cutting edges. However, it fails to describe the clearance angle of the first and second straight cutting edges 309A, 309B constituting the main cutting edge 309.